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The last few years have seen a rapid increase in the number of digital imaging devices produced for radiographic applications. With digitized video/ˆ•image intensifiers, computed radiography (CR), and more recently the advent of flat-panel imagers, an almost bewildering number of choices of digital imaging devices is available. The good news from this rapid progress in digital radiography is that devices are becoming available with image quality superior to that available just a few years ago. The challenge from this proliferation of digital devices is making the difficult decision about which device is most suitable for a particular application. Radiologists will rightly ask, “How can I know which device I should purchase for my clinical imaging needs?” Imaging physicists, on the other hand, will be concerned with verifying the image quality specifications of the various manufacturers, developing appropriate algorithms to extend the utility of the devices, and developing appropriate clinical imaging protocols to take best advantage of the devices' imaging performance. Physicists and radiologists will need to work collaboratively to determine the appropriate applications for these devices, and the performance that may be expected from them. In order to assess the performance of these devices, it is necessary to consider both physical image-quality parameters as well as the observer's perceptual response. Both of these areas are more difficult with digital devices than with screen film. Because the image has been sampled, quantized, and processed, there are a number of changes necessary in traditional measurements of image-quality. Furthermore, because there is an almost unlimited degree of image processing that can be done to the images, it becomes more difficult to gauge observer acceptance of the resulting images. There are a number of physical and observer-based measurements which can be used to gauge image-quality, and these will be considered in this chapter. This chapter is divided into four sections. First, global measures of image-quality will be addressed, using measurements in Cartesian (or image-intensity) space. Second, measures in spatial frequency space will be considered. Third, methods of assessing image processing will be considered, and last, observer assessment will be addressed. Of course, the final stage in assessment of image-quality is the ability of a particular imaging device and clinical protocol to improve diagnostic accuracy. The issue of diagnostic accuracy, while very important, is an entire field of study in itself, and is therefore beyond the scope of this chapter. The interested reader is referred to other chapters in this volume on measuring diagnostic accuracy.
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